Note: I use an 18×13 sheet cake pan.

Pour over flour mixture, and stir lightly to cool.In a measuring cup, pour the buttermilk and add beaten eggs, baking soda, and vanilla.Stir buttermilk mixture into butter/chocolate mixture.Pour into sheet cake pan and bake at 350-degrees for 20 minutes.While cake is baking, make the icing. Melt butter in a saucepan. Add cocoa, stir to combine, then turn off heat. Add the milk, vanilla, and powdered sugar. Stir together.Pour over warm cake. Cut into squares, eat, and totally wig out over the fact that you’ve just made the best chocolate sheet cake. Ever.